1. Technical Field
The present invention relates to a piezoelectric resonator that excites a thickness shear vibration mode and, more particularly, to a piezoelectric vibration element having a so-called inverted mesa structure, a manufacturing method for the piezoelectric vibration element, a piezoelectric resonator, an electronic device, and an electronic apparatus using the piezoelectric resonator.
2. Related Art
In an AT cut quartz crystal resonator, a vibration mode of main vibration to be excited is thickness shear vibration. The AT cut quartz crystal resonator is suitable for a reduction in size and an increase in frequency and assumes a cubic curve having an excellent frequency-temperature characteristic. Therefore, the AT cut quartz crystal resonator is used in various apparatuses such as a piezoelectric oscillator and an electronic apparatus.
JP-A-2004-165743 discloses an AT cut quartz crystal resonator having a so-called inverted mesa structure in which a recessed section is formed in a part of a principal plane to realize an increase in frequency. In the AT cut quartz crystal resonator, a so-called Z′ long substrate having the length in the Z′ axis direction of a quartz crystal substrate larger than the length in the X axis direction is used.
JP-A-2009-164824 discloses an AT cut quartz crystal resonator having a so-called inverted mesa structure in which thick supporting sections (thick sections) are respectively connected to three sides of a rectangular thin vibrating sections and the thick sections are provided in a C shape. Further, a quartz crystal vibrating reed is an in-plane rotation AT cut quartz crystal substrate formed by rotating each of the X axis and the Z′ axis of an AT cut quartz crystal substrate about the Y′ axis in a range of −120° to +60°. The quartz crystal vibrating reed has a structure that secures a vibration region and is excellent in mass production (produced in a large number).
JP-A-2006-203700 and JP-A-2002-198772 disclose an AT cut quartz crystal resonator having an inverted mesa structure in which thick supporting sections are respectively connected to three sides of a rectangular thin vibrating section and the thick sections are provided in a C shape. As a quartz crystal vibration reed, a so-called X long substrate having the length in the X axis direction of a quartz crystal substrate larger than the length in the Z′ axis direction is used.
JP-A-2002-033640 discloses an AT cut quartz crystal resonator having an inverted mesa structure in which thick supporting sections are respectively connected to adjacent two sides of a rectangular thin vibrating section and the thick sections are provided in an L shape. As a quartz crystal substrate, a Z′ long substrate is used.
However, in JP-A-2002-033640, to obtain the L-shaped thick sections, as shown in FIGS. 1C and 1D of JP-A-2002-033640, the thick sections are removed along a line segment α and a line segment β. The thick sections are removed on condition that the thick sections are removed by machining such as dicing. Therefore, a cut surface is subject to damage such as chipping and crack and an extremely thin section is broken. In a vibration region, for example, an unnecessary vibration occurs and a CI value increases, which cause spurious vibrations.
JP-A-2001-144578 discloses an AT cut quartz crystal resonator having an inverted mesa structure in which a thick supporting section is connected to only one side of a thin vibrating section.
JP-A-2003-264446 discloses an AT cut resonator having an inverted mesa structure in which recessed sections are formed to be opposed to each other on both principal planes, which are front and rear surfaces, of a quartz crystal substrate to realize an increase in frequency. JP-A-2003-264446 proposes a structure in which, as the quartz crystal substrate, an X long substrate is used and an excitation electrode is provided in a region where flatness of a vibration region formed in the recessed section is secured.
Incidentally, it is known that, in a thickness shear vibration mode excited in a vibration region of an AT cut quartz crystal resonator, because of anisotropy of an elastic coefficient, a vibration displacement distribution is an elliptical shape having a long axis in the X axis direction. JP-A-2-079508 discloses a piezoelectric resonator that includes a pair of ring-like electrodes arranged symmetrically on both the front and rear surfaces of a piezoelectric substrate and excites thickness shear vibration. A difference between the diameter of the outer circumference and the diameter of the inner circumference of the ring-like electrode is set such that the ring-like electrode excites only a symmetrical zero-order mode and hardly excites other anharmonic high-order modes.
JP-A-9-246903 discloses a piezoelectric resonator in which both the shapes of a piezoelectric substrate and excitation electrodes provided on the front and the back of the piezoelectric substrate are formed in an elliptical shape.
JP-A-2007-158486 discloses a quartz crystal resonator in which both the shapes of both ends in a longitudinal direction (the X axis direction) of a quartz crystal substrate and both ends in the X axis direction of an electrode area semi-elliptical shape and a ratio of the major axis and the minor axis of the ellipse (the major axis/the minor axis) is set to about 1.26.
JP-A-2007-214941 discloses a quartz crystal resonator in which an elliptical excitation electrode is formed on an elliptical quartz crystal substrate. A ratio of the major axis and the minor axis is desirably 1.26:1. However, JP-A-2007-214941 indicates that, when fluctuation of manufacturing dimensions and the like are taken into account, the ratio is practically in a range of about 1.14 to 1.39:1.
JP-UM-A-61-187116 discloses a piezoelectric resonator having a structure in which a cutout or a slit is provided between a vibrating section and a supporting section in order to further improve an energy confinement effect of a thickness shear piezoelectric resonator.
When a reduction in the size of a piezoelectric resonator is realized, deterioration in an electric characteristic and a frequency aging failure are caused by residual stress due to an adhesive. JP-A-9-326667 discloses a quartz crystal resonator in which a cutout or a slit is provided between a vibrating section and a supporting section of an AT cut quartz crystal resonator having a rectangular flat shape. JP-A-9-326667 indicates that it is possible to suppress, by using such a structure, the residual stress from spreading to a vibration region.
JP-A-2009-158999 discloses a resonator in which a cutout or a slit is provided between a vibrating section and a supporting section of an inverted mesa piezoelectric resonator in order to reduce (relax) mount distortion (stress). JP-A-2004-260695 discloses a piezoelectric resonator in which conduction of electrodes on the front and rear surfaces is secured by providing a slit (a through hole) in a supporting section of an inverted mesa piezoelectric resonator.
JP-A-2009-188483 discloses a quartz crystal resonator in which an unnecessary mode of a high-order contour system is suppressed by providing a slit in a supporting section of an AT cut quartz crystal resonator of a thickness shear vibration mode.
JP-A-2003-087087 discloses a resonator in which spurious vibrations are suppressed by providing a slit in a connected section of a thin vibrating section and a thick retaining section, i.e., a residue section having an inclined plane of an inverted mesa AT cut quartz crystal resonator.
In recent years, there is a strong demand for a reduction in size, an increase in frequency, and improvement of performance of a piezoelectric device. However, it has been found that, in the piezoelectric resonators having the structures explained above, a CI value of main vibration, a near spurious CI value ratio (=CIs/CIm, where CIm is the CI value of the main vibration and CIs is a CI value of spurious vibrations; the near spurious CI value ratio is equal to or larger than 1.8 in one example of a standard), and the like cannot satisfy requirements.